1. Field of the Invention
The present invention relates to a hydrogen absorbing alloy electrode used as a negative electrode of an alkali secondary battery such as a nickel-hydrogen secondary battery, a method of fabricating the hydrogen absorbing alloy electrode, and an alkali secondary battery using the hydrogen absorbing alloy electrode, and is characterized in that a hydrogen absorbing alloy used for the hydrogen absorbing alloy electrode is modified, to improve the activity in the early stages of the hydrogen absorbing alloy electrode and the characteristics thereof at low temperature.
2. Description of the Related Art
A nickel-hydrogen secondary battery has been conventionally known as one example of an alkali secondary battery. In the nickel-hydrogen secondary battery, a hydrogen absorbing alloy electrode using a hydrogen absorbing alloy has been generally used as its negative electrode.
Examples of the hydrogen absorbing alloy used for the negative electrode include a hydrogen absorbing alloy having a CaCu.sub.5 -type crystal structure using Misch metal (Mm) which is a mixture of rare earth elements or a Laves type hydrogen absorbing alloy.
In each of the hydrogen absorbing alloys, however, a coating of an oxide or the like is generally formed on its surface by natural oxidation, for example. When a hydrogen absorbing alloy electrode is fabricated using such a hydrogen absorbing alloy, and the hydrogen absorbing alloy electrode is used as a negative electrode of the nickel-hydrogen secondary battery, the activity in the early stages of the hydrogen absorbing alloy is low, and hydrogen gas is not sufficiently absorbed in the hydrogen absorbing alloy. As a result, some problems arise. For example, the capacity in the early stages of the nickel-hydrogen secondary battery is decreased, and the internal pressure of the battery is increased by the hydrogen gas.
Therefore, in recent years, a method of immersing a hydrogen absorbing alloy in an acid solution such as hydrochloric acid, to remove a coating of an oxide on the surface of the hydrogen absorbing alloy has been proposed, as disclosed in Japanese Patent Laid-Open No. 225975/1993.
When the hydrogen absorbing alloy is thus immersed in the acid solution, to remove the coating of the oxide on the surface of the hydrogen absorbing alloy, some active portions appear on the surface of the hydrogen absorbing alloy.
However, the active portions thus appearing on the surface are oxidized again, whereby the activity in the early stages of the hydrogen absorbing alloy is not sufficiently improved, and the hydrogen gas is not sufficiently absorbed in the hydrogen absorbing alloy in the early stages. As a result, some problems still exist. For example, the capacity in the early stages of the battery is low, and the internal pressure of the battery is increased.
Furthermore, in the hydrogen absorbing alloy electrode using the conventional hydrogen absorbing alloy, the electrochemical catalytic capability thereof is not sufficient, resulting in inferior discharge characteristics in a case where it is used under low temperature.